drm/amdgpu: WARN once if amdgpu_bo_unpin is called for an unpinned BO

It indicates a pin/unpin imbalance bug somewhere. While the bug isn't
necessarily in the call chain hitting this, it's at least one part
involved.
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_object.c

@@ -912,7 +912,7 @@ int amdgpu_bo_unpin(struct amdgpu_bo *bo)
 	struct ttm_operation_ctx ctx = { false, false };
 	int r, i;
 
-	if (!bo->pin_count) {
+	if (WARN_ON_ONCE(!bo->pin_count)) {
 		dev_warn(adev->dev, "%p unpin not necessary\n", bo);
 		return 0;
 	}